The present invention relates to a sheet transport device for transporting each sheet to a specified target location. More particularly, the invention is concerned with a sheet transport device for an image-forming apparatus which makes it possible to transfer an image to an exact position even onto a sheet of maximum size having cutoff margins around a maximum image area, as well as with an image-forming apparatus employing the sheet transport device.
Generally, processes performed by an image-forming apparatus using electrophotographic technology are such that an electrostatic latent image corresponding to an image signal, for instance, is formed on a latent image carrier, such as a photosensitive drum, and a toner image obtained by developing the latent image is transferred onto a sheet of paper or other material, directly or indirectly by way of an intermediate image transfer device.
In this kind of image-forming apparatus, the maximum sheet size that can be used is determined by the maximum image area of the latent image carrier like a photosensitive drum on which the latent image is produced. The maximum sheet size thus determined is A3 size as defined in a Japanese Industrial Standard (JIS), for example.
To transfer an image to an exact position on a sheet, the sheet is usually aligned with a specific reference position. For example, this sheet alignment operation is achieved by a leading edge registration method in which the sheet is fed to am image transfer part after its leading edge has been correctly positioned, or by a side edge registration method in which the sheet is fed to the image transfer part after its side edge has been set to a specific side reference position.
The leading edge registration method is associated with a problem that when images are formed on both sides of the sheet, they tend to be incorrectly aligned with each other. This is because the sheet is likely to be fed obliquely in the leading edge registration method. Compared to this, the sheet is always lined up with the side reference position in the side edge registration method. Therefore, the side edge registration method is preferable in that it helps reduce misalignments of images formed on both sides of the sheet.
Also known in the prior art is an oblique feed correction technique used in sheet transport processes. This technique aligns each sheet with a specific side reference position by moving a registration roller, for instance, in a direction perpendicular to a sheet transport direction. Examples of the oblique feed correction technique are described in Japanese Laid-open Patent Publications No. 59-4552, No. 61-249063 and No. 63-185758, and Japanese Patent No. 2632405.
To further improve the performance of this kind of image-forming apparatus, those provided with various aftertreatment devices, such as a stapler, a puncher and a binder, have thus far been proposed.
Under such circumstances, the inventor of the present invention fitted aftertreatment devices like a trimmer to an image-forming apparatus and examined the possibility of providing a high-accuracy printing system. Test results have proved that to obtain a maximum image area equal to JIS A3 size (297 mm wide), for example, the printing system must be able to handle a sheet as large as A3 broad size (320 mm wide), for example, which is larger than the A3 size, and trim the sheet of the A3 broad size after an image has been fixed onto it to produce a print of the standard A3 size.
To meet such requirements, there is no way but to make the maximum image area that can be handled by the image-forming apparatus larger than usable sheet sizes.
To enlarge the maximum image area, however, it is inevitable for the apparatus to become large-sized, and this would result in an increase in product cost. Moreover, development efforts for increasing the maximum image area would be enormous and time-consuming.
The maximum image area of existing image-forming apparatus designed to handle A3 size sheets is naturally the A3 size. Thus, none of the existing image-forming apparatus can handle A3 broad size sheets without extensive design change. Although the A3 broad size is only 23 mm wider than the A3 size, increasing the maximum image area of the existing image-forming apparatus by this amount would involve almost the same man-hours as would be required for developing a new image-forming apparatus. In addition, such a modification would make it necessary to redesign or newly develop almost every component.
Even when the A3 broad size sheet is used, however, a final image is not formed throughout its entire surface area but in the area of the A3 size, because portions of the sheet around the central A3 area where the image is formed serve simply as cutoff margins.
In this situation, the inventor has reached the conclusion that an image-forming apparatus intending to handle the A3 broad size sheet need not necessarily provide a maximum image area as large as the A3 broad size but may be so constructed that it can transfer an image exactly onto the central A3 area of the A3 broad size sheet by using a readily available image-forming module capable of handling the standard A3 size sheet.
The present invention has been made in view of the above circumstances and provide a sheet transport device image-forming module, wherein the sheet transport device makes it possible to transfer an image to an exact position even on a sheet of maximum size having cutoff margins around a maximum image area of the image-forming module. The invention also provides an image-forming apparatus employing the sheet transport device.
According to an aspect of the invention, a sheet transport device comprises a registration/transport member 2 provided in a sheet path upstream of a target location P for correctly positioning a sheet 1 in a sheet transport direction and transporting it toward the target location P, and a sheet alignment mechanism 3 provided in the sheet path upstream of the target location P for moving the sheet 1 in a direction perpendicular to the sheet transport direction to align the sheet 1 to a reference position predefined for each set of sheet information, as shown in FIG. 1.
While the aforementioned construction of the sheet transport device of the invention is applicable to a wide range of sheet transport devices in which the sheet 1 of paper or other material is transported toward the target location P, it is particularly effective when implemented in an image-forming apparatus which requires a high positioning accuracy of the sheet 1 in the sheet transport direction.
According to another aspect of the invention, an image-forming apparatus comprises an image carrier 5 which carries an image T formed on its image transfer part, a sheet transport device 6 which transports a sheet 1 to the image transfer part of the image carrier 5, and an image transfer element 7 which transfers the image T on the image carrier 5 onto the sheet 1 at the image transfer part, the sheet transport device 6 including a registration/transport member 2 provided in a sheet path upstream of the image transfer part for correctly positioning the sheet 1 in a sheet transport direction and transporting it toward the image transfer part, and a sheet alignment mechanism 3 provided in the sheet path upstream of the image transfer part for moving the sheet 1 in a direction perpendicular to the sheet transport direction to align the sheet 1 to a reference position predefined for each set of sheet information, as shown in FIG. 1.
While a typical example of the registration/transport member 2 that can be used in the aforementioned sheet transport device and image-forming apparatus would be a driving roller (registration drive roller) associated with a driven roller (registration idle roller) which is pressed against the driving roller to nip and transport the sheet 1, the invention is not limited to this arrangement. For example, the registration/transport member 2 may be additionally provided with a gate member for temporarily stopping the sheet 1, or other alternative arrangements may be used as appropriate.
Basically, the sheet alignment mechanism 3 is an arrangement for moving the sheet 1 in the direction perpendicular to the sheet transport direction. A characteristic feature of the sheet alignment mechanism 3 is that it aligns the sheet 1 to the reference position predefined each set of sheet information (e.g., size, orientation and type).
In one typical form of the sheet alignment mechanism 3, it utilizes the registration/transport member 2 as a constituent part, for example. Specifically, the registration/transport member 2 is fitted to the sheet alignment mechanism 3 movably in the direction perpendicular to the sheet transport direction, wherein the sheet alignment mechanism 3 moves the registration/transport member 2 from its home position in the direction perpendicular to the sheet transport direction with the sheet 1 nipped by the registration/transport member 2.
In another form of the sheet alignment mechanism 3, it is a sheet-shifting mechanism provided upstream of the registration/transport member 2 with respect to the sheet transport direction, the sheet-shifting mechanism including a movable guide which shifts the sheet 1 toward the reference position before it is nipped by the registration/transport member 2.
In still another form of the sheet alignment mechanism 3 preferable for improving sheet alignment accuracy, it includes an initial alignment mechanism which aligns a side edge of the sheet 1 to an initial side alignment position, and a reference position alignment mechanism which aligns the sheet 1 initially aligned by the initial alignment mechanism to the reference position predefined for each set of sheet information.
In one example of this form of the sheet alignment mechanism 3, the initial alignment mechanism includes an initial side alignment position setting member which defines the initial side alignment position in the direction perpendicular to the sheet transport direction, and an oblique transport member which moves the sheet 1 obliquely toward the initial side alignment position setting member.
According to a preferable method of setting the reference position for each set of sheet information, the sheet alignment mechanism 3 includes a memory storing the reference position predefined for each set of sheet information, and a sheet-shifting mechanism which shifts the sheet 1 in the direction perpendicular to the sheet transport direction to align the sheet 1 to the reference position stored in the memory, for example.
According to a preferable method of shifting the sheet 1 to the reference position, the sheet alignment mechanism 3 includes a side edge position sensor which detects the location of a side edge of the sheet 1, and a sheet-shifting mechanism which determines a side shift amount required for the sheet 1 to reach the reference position based on a sensing signal from the side edge position sensor and shifts the sheet 1 in the direction perpendicular to the sheet transport direction as much as the side shift amount.
To smoothly transport the sheet 1 by the sheet alignment mechanism 3 which utilizes the registration/transport member 2 as a constituent part, it is preferable that the registration/transport member 2 be relieved of its state of nipping the sheet 1 after a force advancing the sheet 1 has been applied to it by a transport member (which corresponds to the image transfer element 7, for example) disposed at the target location P.
Furthermore, to smoothly perform a succeeding sheet alignment operation, it is preferable that the registration/transport member 2 be relieved of its state of nipping the sheet 1 and reset to the home position after a force advancing the sheet 1 has been applied to it by a transport member (which corresponds to the image transfer element 7, for example) disposed at the target location P.
To make it possible to form an image at the center of the width of the sheet 1 in the image-forming apparatus, it is necessary for the sheet alignment mechanism 3 to have the capability of aligning a center line of the width of the sheet 1 with the reference position which is taken at a center line of the width of the image carrier 5.
Especially for forming an image at an exact position when the image-forming apparatus is of a type in which the dimension of the image carrier 5 as measured in the direction perpendicular to the sheet transport direction corresponds to that of a maximum image area, it is preferable that the sheet alignment mechanism 3 align a center line of the width of the sheet 1 with the reference position which is taken at a center line of the width of the image carrier 5 at least when the sheet 1 has a specific blank area around the maximum image area.
Furthermore, to reduce the distance of moving a small-sized sheet 1 widthwise in the sheet alignment operation performed by the sheet alignment mechanism 3 when the image-forming apparatus is of a type in which the dimension of the image carrier 5 as measured in the direction perpendicular to the sheet transport direction corresponds to that of a maximum image area, it is preferable that the sheet alignment mechanism 3 align a side edge of the sheet 1 to a side reference position when the sheet 1 is smaller than the maximum image area. This makes it possible to simplify the construction of the sheet alignment mechanism 3 and decrease its operating time.
Moreover, to prevent a problem (i.e., local deterioration of the image carrier 5) which may potentially occur due to uneven use of a surface area of the image carrier 5 when handling small-sized sheets 1, it is preferable that the sheet alignment mechanism 3 can change the reference position predefined for each set of sheet information.
The operation of the above-described sheet transport device and image forming apparatus is now explained.
As shown in FIG. 2, the registration/transport member 2 is provided upstream of a target location P and transports the sheet 1 toward the target location P to correctly position it.
At the same time, the sheet alignment mechanism 3, provided in the sheet path upstream of the target location P, moves the sheet 1 in a direction perpendicular to the sheet transport direction to align the sheet 1 to a reference position predefined for each set of sheet information.
A sheet 1 (1) smaller than a maximum image area Gmaxis moved from an initial side alignment position SIP and aligned to a reference position a1, whereas sheets 1 (2) and 1 (3) larger than the maximum image area Gmax are aligned to reference positions a2 and a3, respectively.
For the sheets 1 (2) and 1 (3) larger than the maximum image area Gmax, the reference positions are set so that an image corresponding to the maximum image area Gmax is formed in the sheet 1 (2) or 1 (3).
As stated above, the sheet alignment mechanism 3 moves the sheet 1 in the direction perpendicular to the sheet transport direction to align it to a reference position predefined for each set of sheet information in a process of transporting the sheet 1 to the target location P according to the present invention. Accordingly, if optimum reference positions are set for various types of sheets with respect to the maximum image area of a readily available image-forming module, it is possible to exactly transfer an image not only onto a sheet of any size equal to or smaller than the maximum image area but also onto a sheet of a maximum size having cutoff margins around the maximum image area of the image-forming module by using the relevant image-forming module as it is.
It will be recognized that it becomes possible to exactly transfer an image onto various types of sheets including those larger than the maximum image area of an existing image-forming module by using it as it is and just developing a sheet transport device of a new design. Therefore, it is possible to easily construct a high-performance image-forming apparatus provided with such an aftertreatment device as a trimmer without increasing the physical size of the apparatus or developing new components on an extensive scale.